The invention relates to a metal gasket installed between two members, such as a cylinder head and a cylinder block, of an internal combustion engine to seal therebetween. More specifically, the invention relates to a metal gasket formed of a single metal base plate or a plurality of metal base plates, coated with a coating.
When joint surfaces between a cylinder head and a cylinder block (cylinder body) of an engine of an automobile is sealed, a cylinder head gasket is installed therebetween to seal a combustion gas, cooling water and the like.
In view of demand for a light weight and low cost of the engine, a structure of the cylinder head gasket has been shifted to a simple type formed of a single or two metal base plates from a laminated type having a number of metal plates. Due to a few constituent plates, only limited types of materials can be used.
In the laminated type metal plate gasket, sealing devices, such as a bead, grommet and shim, can be combined as desired or provided in double for effective sealing. However, in the single or two plate gasket, a type and a number of sealing devices are limited, so that a simplified sealing device has to be used. Further, the area available for the sealing device is limited with reduction of an engine size.
As shown in FIG. 5, the cylinder head gasket is formed to have a shape of an engine member, such as a cylinder block. The cylinder head gasket includes holes 2 for cylinder bores (hereinafter referred to simply as “cylinder bore 2”); fluid holes 3, 4 for circulating the cooling water and engine oil (hereinafter referred to simply as “fluid holes 3, 4”); and bolt holes 5 for tightening bolts, and the like. Also, the sealing devices, such as beads 12, 13, with respect to the respective holes 2, 3 to be sealed, are provided.
In case a gasket is sealed between the engine members, the sealing properties of the gasket required by the respective holes are greatly different. At the cylinder bore 2, it is required to seal the combustion gas with a high temperature and a high pressure, while at the fluid hole 3, it is required to seal a fluid with a relatively low temperature and pressure.
The cylinder bore 2 is provided with a bead 12 with an arc shape and a narrow width, which is made of a material with elasticity to fit the uneven surface. The cylinder bore 2 is also surrounded by a plurality of bolt holes 5, and bolts are inserted therein and tightened to apply a large surface pressure around the cylinder bore 2.
On the other hand, the fluid hole 3 is provided with the bead 13 to seal therearound. Depending on a structure of the engine, the fluid hole 3 may be positioned outside the area surrounded by the bolt holes 5. In this case, a pressing force of the tightening bolt is applied to only one side of the fluid hole 3, so that the surface pressure becomes small. To solve the problem, a higher bead, a narrower bead, or a pointed bead may be used.
Further, to improve the fitting property between the bead and the metal surface abutting against the gasket, a coating has been used.
For example, in a gasket disclosed in Japanese Utility Model Publication (JITSUKAIHEI) No. 2-16861, a bead in a belt shape is locally provided and, at the same time, an elastic coating film is provided on both surfaces of the metal base plate. In this case, the thickness of the elastic coating film provided on the bead projection side is made thicker than that on the other side. Thus, a strong tightening pressure applied to the top portion of the bead is absorbed by the thick elastic coating film to thereby keep the same tightening surface pressure as that on the other side. Thus, good surface pressure balance can be maintained, and a high torque and durability can be obtained.
However, in the gasket, since a large sealing surface pressure is applied to the top portion of the bead, when the gasket is used for a long time, the elastic coating film covering the bead top portion causes a flow or wear-out. Thus, the sealing surface pressure is substantially reduced to cause the torque reduction.
Also, in a gasket disclosed in Japanese Utility Model Registration No. 2605613, a rubber-like elastic material is coated on both surfaces of a supply member with a bead formed of a metal plate. In the gasket, a rubber hardness of the coating applied to the bead projection side is set higher than that on the other side to suppress the flow of the coating at the top portion of the bead. To set the rubber hardness on the other side lower, a surface pressure per unit area is held at a low value to thereby balance the sealing properties on both surfaces.
In the gasket, the sealing surface pressure is lowered, as a whole, by the coating with the lower rubber hardness applied on the other side. However, the sealing surface pressure on the bead top portion becomes partially extremely high when compared with that of the periphery of the bead by the coating with the high rubber hardness applied on the bead top side. Thus, there has been such a problem that the surface pressure applied to the coating on the bead top portion becomes high, and the flow and torque reduction, which results in a substantial lowering of the surface pressure, can not be prevented.
Further, a metal gasket disclosed in Japanese Patent Publication (TOKKAIHEI) No. 10-103523 includes a first coating layer provided on a bead projection side of a metal plate except for a bead projection portion, and a second coating layer provided on the other side of the metal plate. Since the first coating layer is not applied on the bead projection portion, the flow and the wear-out by repetition of the thermal expansion and contraction of the first coating layer do not occur at the bead top portion to thereby prevent the torque reduction, or reduction of the sealing surface pressure at the bead portion with passage of time.
However, since the second coating layer is applied to the overall surface on the side opposite to the bead projection side, the flow and wear-out of the second coating layer take place at the foot portions of the bead, where the second large sealing surface pressure next to that of the top portion of the bead is generated. Thus, a torque reduction takes place at these portions.
In view of the above problems, the present invention has been made. An object of the invention is to provide a metal gasket having good sealing property and durability, wherein when joint surfaces between a cylinder head and a cylinder block of an engine is sealed by a metal gasket formed of a single or a plurality of metal base plates, the flow and wear-out of the coating provided around the bead are not caused to thereby prevent the torque reduction.
Further objects and advantages of the invention will be apparent from the following description of the invention.